Electricity and energy consumption in data centers is growing at a rapid rate of 12%, based on trends in the industry as of 2006 [1]. This figure includes air conditioning and conventional cooling (i.e., not cryogenic cooling), and in 2006 represented about 1.5% (or about 61 billion kWh) of the electricity used in the U.S. retail sector. An average internet data center consumes about 1-2 kW of power per square meter, with a total capacity of 10-50 MW being installed in new data centers. DC power transmission has the potential for minimizing conversion losses (mainly by reducing reactive power), yielding energy savings of about ˜7-10% [2]. In conventional cooling schemes, every watt of reduction in energy consumption leads to an additional reduction of 0.6 to 2 watts of cooling power. FIG. 1 shows that the cost of powering and cooling servers is rapidly approaching that spent in purchasing and installing new servers [4]. FIG. 2 shows the distribution of energy consumption in a typical data center, with the majority going to the servers, but closely followed by the cooling costs [2]. Including the power conversion and distribution equipment, the energy consumed by equipment other than computers and servers is greater than 50% of the total. FIG. 3 shows a typical data center power structure.
Operating computers and computer chips at cryogenic temperatures, especially at 77 K, has been shown to improve computing speed and to reduce certain losses [3, 6-11]. Much work was done in the early 1990's in this field. Because of the costs of cooling an individual computer, most of this work has ceased. By the time a computer was adapted for cryogenic operation, a newer and more powerful computer, which outperformed even the cryogenically cooled one, became available on the market. However, it has been reported that Moore's Law, which states that the number of transistors that can be placed on a computer chip doubles every two years [12], is reaching limits in both economic and technical viability, and may be obsolete within 5-20 years [13,14]. This is verified by the prevalence of parallel and multi-core processors in today's servers and computers. Thus, cryogenically cooled computers may re-emerge as the fastest and most reliable in time. Cryogenic cooling of computers may also lead to reduced power consumption.
MTECH Labs is a pioneer in the field of Cryogenic Power Conversion (CPC) [19-44]. The key to CPC is the fact that the loss-producing on-resistance or on-state voltage of high-voltage power MOSFETs decreases by a factor of 14 to 35 through cryo-cooling, as shown by MTECH's measurements of FIG. 4. The physics behind this effect is the drastic increase at low temperatures of the majority carrier electron mobility in the drain-drift region of a high-voltage power MOSFET. Other parameters that improve at low temperatures include switching speed, lifetime and reliability (provided the devices are kept cold), and current-carrying capability.
For these reasons, cryogenic DC-DC converters may outperform conventional ones, especially in efficiency, even when the cooling penalty is included. MTECH has also found that replacing conventional transformers with cryogenic inverters would lead to higher efficiencies. As an example, a typical commercial building power system was examined. Conventional transformers consist of either copper or aluminum windings and a permeable iron core. They are rated by temperature rise above the ambient, with typical values of 80° C., 115° C., or 150° C. A study performed by the Cadmus Group, Inc. and funded by the Northeast Energy Efficiency Partnerships, Inc. in 1999 demonstrated that the average load of a transformer in a building was 15% of the rated nameplate capacity [15]. The study looked at various types of buildings, including universities, manufacturing facilities, and office buildings. This study found that a 14-17% average load was fairly consistent across all types of buildings.
FIG. 5 includes data taken from the same study and shows that the best 75-kVA transformers (as of 1999) have efficiencies of 98% at optimal loads (about 35% of the nameplate capacity). However, this drops to about 97% efficiency at 15% of the nameplate capacity (and is much lower for other, less efficient types of standard transformers). FIG. 5 also includes the projected efficiency of MTECH's cryogenic inverters (based on a preliminary design and model). These inverters are semiconductor-based devices intended for integration with DC high-temperature superconducting (HTS) cables. Since the cryo-inverters are based on switches, and do not exhibit the magnetization losses encountered in transformers at low power levels, the efficiencies of these devices increase as the power level decreases. Conventional transformers, on the other hand, require a magnetization current which generates non-negligible core losses even for zero-load conditions. In other words, their efficiency falls drastically at low loads.
Note that the transformer core losses are 300-800 W, power these devices dissipate even in open-circuit conditions under no load. Note also that the loss calculations of MTECH's cryogenic inverter include refrigeration losses of 10 W/W—that is, 10 watts of power are required for a refrigerator to remove 1 watt of power at 77 K, which corresponds to just under 30% of the ideal Carnot efficiency, an attainable number with today's refrigeration technologies.
Assuming a building with 1 MW of power installed, the average power being handled by the transformers as a whole would be about 150 kW (15%). Further assuming that the building is equipped with conventional transformers having an efficiency of about 97% at 15% of nameplate capacity, the losses would then be 4.5 kW. Replacing these bulky and heavy copper/iron transformers with cryogenic inverters operating at an efficiency of 99% or higher at this power level, the losses would be reduced by a factor of three to 1.5 kW. The yearly savings for one such building would be 26,280 kWh (including refrigeration losses). It is especially important to note that these cryogenic inverters have no magnetization losses, as do transformers, and therefore do not exhibit standby losses.
Savings in transmission losses throughout the building are equally as important. Assuming line losses of about 5% (consisting mainly of I2R Joule heating losses and skin effect losses), these would amount to 7.5 kW. By converting the electrical power to DC before transmission and utilizing superconducting cables throughout the building, both the resistive and the skin-effect losses are reduced to almost nothing. This translates into yearly savings of 65,700 kWh. Assuming (conservatively) that the refrigeration losses reduce this by as much as 50% (including the refrigerator inefficiency and heat loads from insulation losses and transitional losses that occur in going from the ambient environment to the cryogenic one, and vice-versa), these savings would still equal 32,850 kWh of energy per year.
Therefore, the total energy savings could be as high as 59,130 kWh for this example of a typical commercial high-rise building. This is the equivalent of saving nearly 30 tonnes (29,565 kg) of coal from being burned every year (per building), thereby keeping more than 54 tonnes of CO2 from polluting the atmosphere.